1. Field of the Invention:
The present invention relates to novel organopoly-siloxane compositions formed principally by intimately admixing diorganopolysiloxane gums, fillers, a group of special additives and organic peroxides. The subject compositions have the advantage, firstly, of being easy to work in the crude state and, secondly, of providing, after hot curing, elastomers having good physical characteristics such as a low compressive set and a high elasticity.
2. Description of the Prior Art:
The introduction of a variety of additives for modifying the properties of diorganopolysiloxane compositions and of the elastomers resulting therefrom is described in numercus documents, the earliest of which date back to the time when heat-curable silicone elastomers were first marketed.
These prior art additives include: (1) Organosilicon compounds possessing SiH bonds, the use of which is described in:
(i) French Pat. No. 1,278,278; the silicone elastomers adhere to themselves and to various substrates. Linear organopolysiloxanes (in fact hydrogenoorganopolysiloxanes) are used in an amount of 4 to 20 parts per 100 parts of dioorganopolysiloxane gums. Reinforcing and non-reinforcing fillers can be added.
(ii) French Patent No. 1,363,839; the silicone elastomers have improved dielectric characteristics and a good adhesion to various substrates. Example 1 of this patent illustrates the use of 0.5 part of a methyl-hydrogenopolysiloxane per 99.5 parts of a methylvinylpolysiloxane gum.
(iii) French Pat. No. 1,377,569; the elastomers adhere to themselves. Hydrogenoorganopolysiloxanes are introduced in an amount of at most 20 parts (in the examples, on the order of 12 to 13 parts) per 100 parts of diorganopoly-siloxane gums; organopolysiloxanes modified by boron are also added in an amount of 5 to 60 parts per 100 parts of diorganopolysiloxane gums.
(iv) Japanese Published application No. 73/093,658; the elastomers have a low compressive set and are only slightly affected by the phenomenon of effluorescence. Hydrogenoorganopolysiloxanes are introduced in a proportion of 0.1 to 10 parts per 100 parts of diorganopolysiloxane gums; calcium derivatives (hydroxide, oxide, hydride, peroxide) are also employed in the same proportions as the hydrogenoorganopolysiloxanes.
(v) British Patent application No. 2,060,667; the elastomers are suitable for the manufacture of gaskets for drive shafts. Hydrogenopolysiloxanes are introduced in an amount of 0.1 to 10 parts per 100 parts of diorganopolysiloxane gums. Two types of fillers are used: a reinforcing silica and diatomaceous earth. Furthermore--page 2, line 34, and page 4, line 9--the use of fluorinated polymer is suggested. (2) Organofluorinated polymers in the form of powders or colloidal materials, the use of which is described in:
(i) U.S. Pat. No. 2,710,290; the elastomers have very good mechanical properties, the tear strength in particular being high. Polytetrafluoroethylene powder having a particle diameter of 0.15 to 100.mu. is introduced in an amount of 1 to 20 parts per 100 parts of the diorganopolysiloxane gum.
(ii) U.S. Pat. No. 4,010,136; the elastomers have a smooth and uniform surface. A polytetrafluoroethylene powder having an average particle diameter of less than 1000.mu. is introduced in an amount of 0.3 to 1 part per 100 parts of a mixture formed from 100 parts of diorganopolysiloxane gums and 15 to 25 parts of reinforcing silicas.
(iii) European Published application No. 51,369; the elastomers are resistant to oils and hot hydrocarbon-based fuels. Polytetrafluoroethylene powder having an average particle diameter on the order of 30 to 500.mu. is introduced in an amount of 0.8 to 5 parts per 100 parts of a mixture of fluorinated silicone gums and reinforcing silicas. (3 ) Alkoxysilanes each possessing an acroyloxy or methacroyloxyalkyl group bonded to the silicon atom, the use of which is described in:
(i) U.S. Pat. No. 3,379,607; a compound such as methacroyloxypropyltrimethoxysilane of the formula CH.sub.2 .dbd.C(CH.sub.3)COO(CH.sub.2).sub.3 Si(OCH.sub.3).sub.3, which is known for improving adhesion, is intimately mixed with a polysiloxane elastomer (lines 28 to 33, column 1).
(ii) Japanese Published application No. 75/061,450; polyester fibers treated with methacroyloxypropyltrimethoxysilane are used as fillers in silicone compositions which cure under the action of heat to give elastomers.
(iii) Japanese Published application No. 78/118,452; Example 1 shows, by way of comparison, the introduction of 1.5 parts of methacroyloxypropyltrimethoxysilane per 100 parts of a dimethylpolysiloxane gum. The compositions provide elastomers which adhere strongly to various substrates.
(iv) U.S. Pat. No. 4,202,812; acroyloxy (or methacroyloxy) alkylalkoxysilanes are introduced in an amount of 0.08 to 0 3 part per 100 parts of a silicone rubber base, associated with 25 to 300 parts of non-reinforcing fillers. The elastomers have improved tensile properties. (4) Boron derivatives or the reaction product of such derivatives with organopolysiloxane polymers, the use of which is described in:
(i) French Pat. No. 1,130,806; the silicone compositions are easy to process. The boron derivatives, such as boric acid, boric anhydride or alkyl borates, are added in an amount of 0.005 to 0.9 part per 100 parts of diorganopoly-siloxane gums.
(ii) U.S. Pat. No. 3,070,560; the compositions are easy to work and they do not stick to the mixing mills and the arms of the kneaders. Boron derivatives, such as alkyl borates, boric acid and anhydride and silyl borates, are introduced in an amount of 0.015 to 0.25 part per 100 parts of diorganopoly-siloxane gums. Alpha, omega-di(hydroxy)diorganopolysiloxane oils having at least 1% by weight of OH groups are also present.
(iii) French Pat. No. 1,248,776; the elastomers are self-adhesive. They contain from 0.5 to 50 parts, per 100 parts of diorganopolysiloxane gums, of a boron-containing polysiloxane derived from the condensation of an alkoxypolysiloxane resin with a complex formed from boric acid and a hydroxyl compound.
(iv) French Pat. No. 1,377,569 (noted hereinabove, in paragraph 1); the boron-modified organopolysiloxanes are prepared by reacting approximately 3 to 30 parts of boron derivatives (such as alkyl borates, boric acid and anhydride, alkali metal and alkaline earth metal borates, boron hydrides) with 100 parts of organopolysiloxanes under the action of heat, in the presence or absence of a catalyst such as ferric chloride. Examples of appropriate organopolysiloxanes are di(hydroxy)dimethylpolysiloxane polymers, octamethylcyclotetrasiloxane and the hydrolysis products of dimethyldichlorosilane, where the latter may or may not be mixed with methyltrichlorosilane.
Thus, it is apparent from the foregoing that properties which are frequently valuable but sometimes disconcerting, such as the adhesion of the elastomers to themselves, can be obtained using one of the additives from the group consisting of hydrogenoorganopolysiloxanes, boron derivatives, acroyloxy (or methacroyloxy) silanes and organofluorinated polymers.
However, the manufacturers and users of silicone elastomers are still seeking materials which possess a set of constant, well-defined properties (and not all of the properties referred to above), irrespective of the quantities employed and the nature of the main constituents of the compositions which cure to give silicone elastomers.
In particular, serious need exists in this art for responsive compositions which do not stick to the rollers of the mixing mills and which do not develop a structure on standing. As regards the ultimate users, they would like more especially to process elastomers having good tensile characteristics, a low compressive set, a high resilience and a good heat stability, these properties applying both to opaque elastomers and to translucent and even transparent elastomers.
The state of this art is conspicuously devoid of suggestion of means to be used in order to achieve the results desired by both the manufacturers and users of silicone elastomers.